Gaymar Industries, Inc. is the assignee of this application, and U.S. Pat. No. 5,875,282 (the “'282 patent”). The '282 patent is directed to an apparatus for warming blood and other fluids to a desired temperature prior to introduction into a patient. In particular, the blood and other fluids flow through a counterflow fluid bag having (1) a flat counter flow pathway which (a) provides high flow performance and (b) ensures uniform, gradual and energy efficient fluid warming and (2) a pair of guide rails. The warming device contains a pair of opposed, identical heater elements separated by a receptive slot that has two guide slots, one on each side of the receptive slot. The guide rails align the bag in the warming apparatus so the bag is spaced and located precisely between the two heater elements. The pair of guide rails, integrated into the sides of the bag, are parallel to one another and provide sufficient rigidity for easy insertion of the bag into the warming apparatus. The blood warmer may also incorporate a microprocessor for precise control of the electric current provided to the heater. The microprocessor can be adjusted to have a set point ranging between ambient temperature and 60° C. and a predetermined shut off temperature point, for example, 43.5° C. Fluid temperature is measured by a RTD sensor—and in a later embodiment with a thermistor—with a thin dielectric surface layer in contact with the bag and located proximal to the fluid outlet. By monitoring the temperature of the fluid at the outlet of the bag, the temperature controller can compute and, is suppose to, provide a visual display of the fluid temperature. In one embodiment, the sensed output temperature is an input parameter to the controller of the heater elements. The device efficiently warms the fluid to a range between and including ambient temperature and 60° C. for anticipated input to a human.